Regular Expressions

Most Regular Expression (Regex) implementations are based on or bare similarities to the PCRE library. The purpose of Regular Expressions are normally to replace or extract bits of data from a larger string. The syntax of its usage will vary between different languages (eg the functions or constructs that let you use Regex in Perl, PHP, PowerShell, etc), but the syntax of the actual Regex will generally be consistent. The secret to creating good Regex is good and thorough testing for things that shouldn't match as much as should match.

The following provides a run through of the general syntax (its not exhaustive, its a bit of a work in progress, but contains most of the common stuff).

See the Regex Examples page for some real world examples.

Metacharacters

Metacharacters define how an item should match (or not)

Character	Meaning	Example
\	Escape
^	Match the beginning of the line	^dog matches dog but not big dog
$	Match the end of the line	dog$ matches dog but not dog run
.	Match any character	do. matches do, dog but not dogs
(...)	Grouping, match entire set ...
[...]	Bracketed Character class, any character contained in set ...	[fld]og* matches fog, log, or dog only
|	Or, alternation	d|fog matches dog or fog
?=	Lookahead	dog(?=,) matches the dog in dog, but won't find a match in dog
?<=	Look behind	(?<=big )dog matches the dog in big dog but won't find a match in dog

Quantifiers

Quantifiers define how many times the preceding item should be matched.

Character	Meaning	Example
*	Match 0 or more times	dog* matches do, dog, dogg, or doggg, and so on
+	Match 1 or more times	dog+ matches dog, dogg, or doggg, and so on; but not do
?	Match 0 or 1 times only	dog? matches do or dog only
{n}	Match exactly n times	dog{2} matches dogg only
{n,}	Match n or more times	dog{2,} matches dogg, or doggg, and so on
{n,m}	Match n up to m times	dog{2,4} matches dogg, doggg, or dogggg only

Character Classes

Character Classes define a set of characters that should be matched

Sequence	Matches	Example
[...]	Any character contained in set ...	[fld]og* matches fog, log, or dog only
[:...:]	Any character defined by POSIX class ...
\w	Any word character (alphanumeric and underscore)
\W	Any non-word character
\s	Any white-space character
\S	Any non-whitespace character
\d	Any decimal digit character
\D	Any non-decimal digit character

POSIX Classes

POSIX classes are generally not supported by Microsoft implementations.

Class	Matches	Equivalent to
alpha	Any alphabetical character	[A-Za-z]
alnum	Any alphanumeric character	[A-Za-z0-9]
ascii	Any ASCII character
blank	A space or tab
cntrl	Control characters
digit	Any decimal digit	[0-9] or \d
graph	Any graphical/visible character
lower	Any lower-case alphabetical character	[a-z]
print	Any printable character, including a space
punct	Any punctuation character
space	Any white-space character
upper	Any upper-case character	[A-Z]
word	Any word character	[A-Za-z0-9_]<\code> or \w
xdigit	Any hexadecimal digit	[0-9a-fA-F]